The present invention relates to a feeder for feeding solid material at a predetermined spacing and at a predetermined velocity.
The feeder is particularly for feeding and discharging discrete objects one by one, i.e. a single particle feeder. However, the feeder can also be used for feeding groups of a small number of objects at a predetermined spacing and velocity, or for dividing up a stream of small particles and feeding quantities of the particles at a predetermined spacing and velocity.
The invention was particularly designed for feeding gem stones such as diamonds, for positioning after an accelerator which provides single feed at an approximately synchronous rate but with the directions of the individual stones differing slightly and the gaps between individual stones differing slightly. The stones may for instance be travelling vertically at about 15 per second at a speed of 1-3 m/s. The stones may for instance be being passed to a machine for weighing them or sizing them or otherwise examining them. It is desirable to improve the accuracy of the spacing, direction and velocity when using high speed apparatus.
For example, the gem stones may have a diameter of about 5 mm and be projected vertically downwards through an 18 mm diameter hole which is 200 mm below the release point from the feeder--the 200 mm is to permit the incorporation of detector assemblies etc. The stones themselves can behave erratically as they may for instance be in the forms of platelets with a somewhat aerofoil cross-section, and also air currents or windage can have a significant effect during the 200 mm vertical travel. It is therefore important that the feeder itself should be as accurate as possible.
The Invention
In accordance with the present invention, the feeder comprises a series of spaced containers which in a feed section descend substantially vertically through a substantial distance, at a predetermined spacing and at a predetermined velocity, each successive container being arranged to receive the solid material as it descends and to discharge the material at the lower end part of the feed section.
As the containers are descending in the feed section, the solid material will be accelerating under gravity when it reaches the containers; the feeder is arranged so that the velocity of descent of the containers is roughly equal to that of the solid material at the top of the feed section and the solid material catches up with the respective container and settles down in the container, i.e. becomes stationary relative to the container; the substantial descent allows this, and within reason, the greater the length of the feed section, the more accurate the feed. If a succession of spaced single objects are being fed, the spacings should be roughly matched at the top of the feed section to avoid a double feed or nipping an object between the two parts of a container (if the containers are in this form). However, there is significant latitude and the spacing of the feed into the feeder can for example have a range of .+-.1/4 the spacing of the containers.
In theory, it is possible to use a one-piece container, but it is important not to impose any lateral forces on the solid material at the bottom of the feed section so that the solid material goes straight on down (e.g. within a vertical axis cone of 5.degree. half angle); thus the container must be arranged to come away as cleanly as possible from the solid material at the bottom of the feed section. In general, it is preferred that the bottom of each container should open in order to discharge the solid material, and this can be arranged by having the bottom (at least) of each container formed of at least two parts which separate in order to open the bottom--three or more parts are possible and four parts are preferred if high accuracy is required.
To avoid applying lateral forces, the bottom of each container should open generally downwards, and it is desirable that, at least after an initial movement, the bottom should accelerate downwards faster than gravity. This can be arranged by tipping the respective container part down from its normal relationship to say a carrying belt, at the bottom of the feed section. This can be done if the container parts are carried on belts by using small ligatures to tip the container parts down, or by using camming. The same principles can be applied to three or four container parts, to reduce or avoid sliding movement between the container parts and the solid material. The four container parts could be arranged to be carried on a single belt or on two belts.
The containers should provide a pocket which is large enough to stop the solid material bouncing out. Particularly if the solid material is in the form of discrete objects, there may be some tendency to bounce, and this can be reduced by making the containers of flexible material so that they are a little compliant.